One option for alwaysUseGithubRepo may be to make it renderable, so you could implement per-builder configuration with
  alwaysUseGithubRepo=Interpolate("%(prop:github_repo_owner)s/%(prop:github_repo_name)s")
but it would leave lots of room for other needs.  This also allows a callback (a custom renderable) for users who need the ultimate flexibility.